This invention is generally related to a system for inspecting the heat exchanger tubes of a steam generator, and is specifically concerned with a multiple-unit probe pusher which can drive any one of a plurality of probes independently from one another and can reliably guide any one of the driven probes to a selected heat exchanger tube.
Probe pushers have seen extensive use in steam generator service applications. One application for probe pushers has been in the eddy current inspection of the steam generator heat exchanger tubing, which checks for faults or failures in the tubes. In eddy current inspection processes, the probe pusher is mounted outside of the steam generator so as to drive an eddy current probe through each tube to be inspected. During this operation, the steam generator and nuclear reactor are shutdown, and many hundreds of heat exchanger tubes are inspected. These eddy current probes have a limited useful life, and when an inspection probe fails, the inspection process must be stopped and a platform worker will have to be sent into the containment area to replace the defective probe. Currently-used eddy current probe pushers include means to drive a single eddy current probe through a steam generator heat exchange tube; but when failure of the probe occurs, a worker must remove the inspecting probe from the probe pusher and replace it with a new probe before the inspection operation can continue. These single drive probe pushers include a split housing that must be opened by the worker to remove and insert the inspection probes, which in turn increases service time, generator downtime (which typically costs the utilies over $600,000 per day in lost revenues) and radiation exposure to maintenance personnel. This is a significant disadvantage, particularly when one considers the fact that over three thousand tubes must typically be inspected in a single, walk-along maintenance operation, and that one or more inspection probes usually fail in each eight-hour shift.
In a different application of probe pushers in a steam generator of a nuclear reactor plant, it has been known to use a multiple-unit probe pusher as a positioning device for inserting and removing a plurality of heat treat stress relief probes all at once in a like multiple number of steam generator tubes for stress relief treatment. Such stress relief heater probes are fundamentally different from inspection probes such as eddy current probes because once the heater probes are positioned by the probe pusher next to the point on the tube to be treated, they, remain in place during the treatment of the tube. Hence the probe pusher is used only as a positioning device prior to the treatment of the tube. In contradistinction, eddy current probes must be driven along the longitudinal axis of the tube while the inspection operation is conducted, and the speed of inspection is directly related to the speed the probe is moved through the steam generator tubes.
The aforementioned probe pusher for positioning a multiple number of heater probes is designed to speed up the process of treating a large number of tubes of a steam generator by actually treating as many tubes with as many heater probes that the probe pusher can position in the tubes at the same time. Thus, stress relief treatment is expedited by treating many tubes in parallel at once. In order to accomplish the above, this known probe pusher includes a plurality of drive paths defined within a probe pusher housing, wherein each path includes a system of drive rollers and clutches between an inlet and an outlet of the housing for each of the heater probes to follow through. The clutches and rollers are mounted on shafts driven by a single drive shaft and the clutches are independently actuable so that each drive path can be operated independently to position heater probes.
While this prior art multiple-unit probe pusher is well adapted for use with heater probes, it is not well adapted for use with inspection probes. For example, upon the failure of any one of the probes being manipulated within a tube there is no simple way to replace the probe that becomes defective in operation without interrupting the operation of all of the probes, as all the probes are connected to flexible cables that are wound and unwound from a single drum. Additionally, this prior art probe pusher is not designed to quickly move its probes along the longitudinal axis of the tubes being treated. Instead, it moves its respective probes at a relatively slow speed since the purpose of such longitudinal movement is only to position the heater probes adjacent to a desired region of the tubes which require heat treatment.
Clearly, there is a need for a multiple probe pusher device and system that affords rapid replacement of defective probes with little or no interruption in the operation of the other probes, and which is capable of rapidly positioning the probes in the tubes and quickly moving them along the length of these tubes. Such a device would overcome the aforementioned shortcomings of increased downtime and radiation exposure while being able to adequately perform an inspection process such as eddy current inspection.